This invention relates to a steering power control device for use in a power steering system. More particularly, the invention relates to a novel steering power control device which varies the steering power in response to the combined values of vehicle load and vehicle speed.
The power steering system gives an auxiliary force to the turning effort applied to a steering wheel and it facilitates the operation of the steering wheel. There are known several kinds of power steering system such as integral type, semi-integral type and linkage type systems. The linkage type power steering systems are further divided into two kinds, that is, the combined type system in which a power cylinder and a control valve are integrally formed and the separate type system in which the above two kinds of elements are formed separately. In order to give the feeling of resistance to the driver, any one of these conventional power steering systems is inevitably provided with a reaction force mechanism which transmits a reaction force to the steering wheel in response to the intensity of the steering output power. In one of the known power steering system, the intensity of the reaction force which is applied to the steering wheel is determined according to the intensity of the auxiliary force of the power steering. In this type, the intensity of the auxiliary force is regulated by the pressure or the quantity of a fluid which is introduced into the working chamber of the power steering. In other words, the larger is the pressure or the quantity of the fluid that is introduced into the working chamber, the larger becomes the ratio of the auxiliary force and the smaller becomes the reaction force to the steering wheel, accordingly.
Further, in recent years, in order to provide safety in high speed running, several kinds of pressure or flow control means have been proposed so as to increase the reaction force applied to the steering wheel with the increase of the vehicle speed even when the ratio of steering output power (ratio of auxiliary force) is the same. In any of such improvements in power steering systems, however, the speeds of the vehicles are detected so as to control the reaction forces applied to the steering wheels in response to the speeds of vehicles, and any factor other than the vehicle speed is not taken into consideration.
Therefore, in a motor truck in which the weight of a load varies greatly between a loaded state and an unloaded state, when several elements are set so as to obtain a proper steering output and a proper reaction force to a steering wheel in a loaded state, the necessary force to operate the steering wheel becomes too light in an unloaded state, especially, the driving stability in a high speed and unloaded condition becomes quite worse. To the contrary, when the several elements are set to operate satisfactorily in an unloaded state, the resistance in the steering operation during a loaded state becomes too heavy.